The present invention relates to radio frequency (RF) instrumentation, and more particularly to a tracking generator with an internal vector modulation source that is controlled directly by a host RF instrument to perform complex measurements on RF devices.
A tracking generator uses replicas of internal local oscillators of an RF measurement instrument, such as a spectrum analyzer, to create a signal that is at the same frequency that the RF measurement instrument is measuring. The tracking generator is generally provided as an option to the RF measurement instrument. By using the internal local oscillators the tracking generator tunes with a receiver in the RF measurement instrument. In conventional form the tracking generator provides a tuning continuous wave (CW) source that is used to sweep filters, amplifiers and other devices under test (DUTs) as a “poor man's” network analyzer. When used with the tracking generator the RF measurement instrument displays the frequency response of the DUT. FIG. 1 shows how the tracking generator works in conjunction with the receiver and a controller in a representative RF measurement device, where replicas of the internal local oscillators LO1 and LO2 are provided by the receiver to the tracking generator.
Some recent RF measurement instruments have provided an external vector modulation source to generate In-Phase and Quadrature-Phase (I/Q) inputs to the instruments. These allow a user to place an arbitrary modulation on the CW output of the tracking generator. However when using such an external modulator, the RF measurement instruments require re-calibration whenever the external modulator is exchanged or re-cabled. Also the external modulator is not controllable by the RF measurement instrument. Another disadvantage is that equalization of the tracking generator is not possible.
What is desired is improved measurement accuracy when using I/Q modulation of a CW test signal for measuring RF characteristics of a device under test.